In flyback converters, high power density and high efficiency may be achieved by employing zero-voltage switching (ZVS) techniques. One of the ZVS techniques, for example, is to discharge an output capacitor of a primary power switch before the primary power switch is turned on by a control signal, thus switching-on loss of the primary power switch is eliminated, and the efficiency is then improved. However, the conventional implementation of ZVS techniques may require a relatively complicated circuit structure and raise an issue of high cost. Accordingly, a simple solution of ZVS for flyback converters is needed.